Preparation and study of nickel based magnetic thin films

Abstract

Thin films of nickel and nickel based alloys with composition NiJOo-xCo{xx = 5, 10, 15 and 20f with and without external field have been prepared by evaporation technique using Edward 306E. High vacuum was maintained during the deposition, the pressure being of the order 10-6 Torrjust before the beginning of the evaporation. For the preparation of samples with induced anisotropy, samples of pure Ni and Ni-Co alloys were deposited onto quartz substrate with an applied field of 500 Oe. A special electromagnet, used within the vacuum chamber, was designed and fabricated locally. The thickness of the samples were controlled by varying the. time of evaporation, the heating current and the amount of powder contained in the tungsten basket. The effect of the variables on thickness was controlled empirically by trial and error and the thicknesses was determined by Tolansky interference method. For characterization of the samples; small angle X-ray diffraction, scanning electron microscopy and secondary ion mass spectroscopy have been carried out. X-ray diffraction shows that the deposited cluster have the fcc structure with the replacement of nickel atoms by cobalt atoms in the case of Ni-Co alloys, the cluster size being about 10A. The composition is found to be homogeneous and identical with that of the starting alloys as found from SIMS. The ferromagnetic phase transition temperature of the samples with varied thickness and compositions have been determined by resistivity anomaly, which shows that the transition VII temperature decreases with decreasing thickness. The magnetic anisotropy of the samples are determined by using a specially designed torque magnetometer, where proportional integrating and differentiating circuit is used with a sensitivity of 1(J1O Nm. The transition temperatures are also measured from the temperature dependence of the magnetic anisotropy which agree quite well within the experimental error, when the difference in the methods of measurements in the two systems are taken into account. Since the samples were too thin for the sensitivity of our vibrating sample magnetometer, only sample with highest thickness could be studied which shows saturation magnetization to be 92.84 emu/ gm at room temperature for Ni9sCOs. The thickness dependence of transition temperature, as determined by resistivity anomaly, shows that in the limit of two dimensions, where thickness should be vanishingly small, ferromagnetic ordering most likely cease to exist.